Concentrations are expressed as the average of annual mean concentrations. Up to 3-year gaps of missing values have been interpolated or extrapolated. Only complete series with no missing values after this interpolation/extrapolation are included. The number of river monitoring stations included in the analysis per geographic region is noted in brackets. BOD7 data (Estonia, Finland, Lithuania (1996-2009), Latvia (1996-2001)) has been recalculated into BOD5 data.